Improvement of residual compressive strength and spalling resistance of high-strength RC columns subjected to fire

This paper investigates the spalling properties of high-strength concrete in order to improve the residual compressive strength and spalling resistance in specimens subjected to 3 h of unloading fire conditions. This study consists of three series of experiments with eighteen different specimens varying in fiber type and content, finishing material and simultaneous fiber content and lateral confinement. They were fabricated to a 300 × 300 × 600 mm mock-up size. Results of the fire test showed that the control concrete was explosive, while the specimens that contained more than 0.1 vol% of polypropylene (PP) and polyvinylalcohol (PVA) fibers were prevented from spalling. One specimen, finished by a fire endurance spray, exhibited even more severe spalling than the control concrete. The specimen containing 0.1 vol% of PP fiber and using a confining metal fabric at the same time, showed the most effective spalling resistance; in particular, the residual compressive strength ratio was even higher than that of the control concrete before the fire test. It was demonstrated that adding fibers in concrete prevented the spalling occurrence and confining metal fabric around the main bars of concrete specimens can secure the strength of structures during the conditions of elevated temperature.     

Correlations among mechanical properties of steel fiber reinforced concrete

In this paper, applicability of previously published empirical relations among compressive strength, splitting tensile strength and flexural strength of normal concrete, polypropylene fiber reinforced concrete (PFRC) and glass fiber reinforced concrete (GFRC) to steel fiber reinforced concrete (SFRC) was evaluated; moreover, correlations among these mechanical properties of SFRC were analyzed. For the investigation, a large number of experimental data were collected from published literature, where water/binder ratio (w/b), steel fiber aspect ratio and volume fraction were reported in the general range of 0.25–0.5, 55–80 and 0.5–2.0%, respectively, and specimens were cylinders with size of Φ 150 × 300 mm and prisms with size of 150 × 150 × 500 mm. Results of evaluation on these published empirical relations indicate the inapplicability to SFRC, also confirm the necessity of determination on correlations among mechanical properties of SFRC. Through the regression analysis on the experimental data collected, power relations with coefficients of determination of 0.94 and 0.90 are obtained for SFRC between compressive strength and splitting tensile strength, and between splitting tensile strength and flexural strength, respectively.     

Strengths and flexural strain of CRC specimens at low temperature

Crumb rubber concrete (CRC) is made by adding rubber crumbs into conventional concrete. This study undertakes an experimental study on the cubic compressive strength, axial compressive strength, flexural strength and splitting tensile strength of CRC specimens at both ambient temperature 20 °C and low temperature ?25 °C. The flexural stress–strain responses were also recorded. The averaged size of rubber crumbs used in the study is about 1.5 mm. Four levels of rubber contents are investigated, which are 0%, 5%, 10% and 15% by volume, respectively. The mix design aimed at 40 MPa of compressive strength and 100 mm of slump for all the CRC specimens. The results show that CRC increases its magnitude in strengths when temperature decreases, which is similar to the case of conventional concrete, but still exhibits ductility in low temperature. The conclusion from this study is that CRC may be more beneficial in its application in low temperature environments than in ambient temperature environments.     

Strength and deformability of waste tyre rubber-filled reinforced concrete columns

This study aims to investigate the efficiency of waste tyre rubber-filled concrete to improve the deformability and energy absorption capacity of RC columns by considering different concrete compressive strength, size of waste tyre rubber particles and rubber content. Twelve column specimens were tested using concrete of compressive strength 24 and 28 MPa mixed with 0.6 and 1 mm tyre rubber particles. For each concrete batch, 27 control specimens were prepared to examine the concrete properties. Using waste tyre rubber-filled concrete leads to a slightly lower compressive strength and modulus of elasticity, but the curvature ductility can increase up to 90%. It is concluded that this type of concrete can offer good energy dissipation capacity and ductility, which makes it suitable for seismic applications.     

Permeability of high-performance concrete subjected to elevated temperature (600 °C)

Permeability is one of the most important parameters to quantify the durability of high-performance concrete. Permeability is closely related with the spalling phenomenon in concrete at elevated temperature. This parameter is commonly measured on non-thermally damaged specimens. This paper presents the results of an experimental investigation carried out to study the effect of elevated temperature on the permeability of high-performance concrete. For this purpose, three types of concrete mixtures were prepared: (i) control high-performance concrete; (ii) high-performance concrete incorporating polypropylene fibres; and (iii) high-performance concrete made with lightweight aggregates. A heating–cooling cycle was applied on 160 × 320 mm, 110 × 220 mm, and 150 × 300 mm cylindrical specimens. The maximum test temperature was kept as either 200 or 600 °C. After the thermal treatment, 65 mm thick slices were cut from each cylinder and dried prior to being subjected to permeability test. Results of thermal gradients in the concrete specimens during the heating–cooling cycles, compressive strength, and splitting tensile strength of concrete mixtures are also presented here. A relationship between the thermal damage indicators and permeability is presented.     

The effect of thiosemicarbazide on corrosion resistance of steel reinforcement in concrete

This study describes a laboratory investigation of the influence of thiosemicarbazide (TSC) on the corrosion of reinforcing steel and the compressive strength of concrete. The effect of TSC on the corrosion resistance of steel reinforced concrete was evaluated by carrying out electrochemical tests in NaCl and NaCl + TSC solutions for 60 days. Polarisation resistance (R_p) values of TSC added reinforced concrete were much higher than those without TSC. Similarly, AC impedance spectra revealed that the resistance of TSC mixed electrodes were also quite higher than those without. The compressive strength of concrete specimens containing TSC was measured and an increase of 20–25% was observed.     

Assesment of clay bricks compressive strength using quantitative values of colour components

This study was conducted to assess the relationships among firing temperature, colour components and compressive strength of bricks. Lightness (L*) and chromaticity (a* and b*) of 10 replicated brick samples fired at temperatures 700–1050 °C in steps of 25 °C under free access of air, were measured with a colorimeter, which uses an L* a* b* colour space. Increasing firing temperature significantly increased the compressive strength of bricks. The values of L* slightly increased with firing temperature up to around 800 °C then decreased as temperature increased further. The values of b* and a* increased with increasing firing temperature up to around 900 °C then rapidly decreased with further increases in firing temperature. A negative relationship occurred between each of L*, a*, and b* and compressive strength. Compressive strength was adequately described by colour components of L* and b* by linear regression equations (R² = 0.87 for L*, and R² = 77 for b*). However, the relationship occurred between a* and compressive strength was quite poor. It was concluded that the numerical values of colour components of L* and b* may be used to predict and judge the compressive strength of bricks. However, the method can not be generalized before its calibrated with different raw materials under different firing conditions.     

Modeling mechanical performance of lightweight concrete containing silica fume exposed to high temperature using genetic programming

In this study, the mechanical performance of lightweight concrete exposed to high temperature has been modeled using genetic programming. The mixes incorporating 0%, 10%, 20% and 30% silica fumes were prepared. Two different cement contents (400 and 500 kg/m³) were used in this study. After being heated to temperatures of 20 °C, 200 °C, 400 °C and 800 °C, respectively, the compressive and splitting tensile strength of lightweight concrete was tested. Empirical genetic programming based equations for compressive and splitting tensile strength were obtained in terms of temperature (T), cement content (C), silica fume content (SF), pumice aggregate content (A), water/cement ratio (W/C) and super plasticizer content (SP). Proposed genetic programming based equations are observed to be quite accurate as compared to experimental results.     

Modeling of some properties of the crushed tile concretes exposed to elevated temperatures

In this study, artificial neural network (ANN) and fuzzy logic (FL) models have been developed for predicting the compressive strength (f_c) and dynamic modulus of elasticity (E_d) of the crushed tile concretes (CTC) exposed to elevated temperatures. Some relationships are established between chosen inputs and outputs by developing and testing a multi-layered feed forward ANN and FL trained with the back-propagation algorithm. First of these relationships is established between the outputs as f_c of CTC after being exposed to elevated temperatures and the inputs as exposed temperature (T), crushed tile aggregate (CT) and crushed stone II (CSII) contents of concrete. The second one is the relationship between E_d of concretes and the same inputs. In this aim, concrete specimens are produced by CT replacing 16–31.5 mm coarse aggregate at the ratios of 0%, 10%, 25%, 50%, 75% and 100%. Concrete specimens are exposed to 20, 150, 300, 400, 600, 900 and 1200 °C high temperatures corresponding TS EN 1363-1 after an initial 28 day curing period. After heating, the specimens are slowly air-cooled to the room temperature and then E_d and f_c of concretes were determined. Experimental results are also predicted by constructing models in ANN and FL methods. In the models, the training and testing results have shown that ANN and FL methods have strong potential for predicting the f_c and E_d of crushed tile concretes exposed to elevated temperatures.     

Parameter optimization on compressive strength of steel fiber reinforced high strength concrete

This paper illustrates parameter optimization of compressive strength of steel fiber reinforced high strength concrete (SFRHSC) by statistical design and analysis of experiments. Among several factors affecting the compressive strength, five parameters that maximize all of the responses have been chosen as the most important ones as age of testing, binder type, binder amount, curing type and steel fiber volume fraction. Taguchi analysis techniques have been used to evaluate L₂₇ (3¹³) Taguchi’s orthogonal array experimental design results. Signal to noise ratio transformation and ANOVA have been applied to the results of experiments in Taguchi analysis. The confirmation runs were conducted for the optimal parameter level combination, which is obtained from the results of the above methodologies. The maximum compressive strength has been observed as around 124 MPa. By using the optimal parameter level combination, the direct tensile strength and flexural strength tests have been conducted. The mean values at the age of 28 days are obtained as 7.5 MPa and 13 MPa respectively. In this study, it is clearly demonstrated that all main factors except steel fiber significantly contribute to the compressive strength of steel fiber reinforced high strength concrete, yet age and binder type are the most significant contributors.     

Impact of melting and burnout of polypropylene fibre on air permeability and mechanical properties of high-strength concrete

This study intends to investigate the impact of high temperature, melting and burnout of Polypropylene Fibre (PP fibre) on mechanical properties, pore size distribution and air permeability of high strength concrete. The specimens were high-strength concrete with 120 MPa strength produced with a water-binder ratio of 20%. To examine the effects of melting and burnout of the PP fibre, the experiment was conducted using two mixtures. One mixture contained 1.5 kg/m³ of PP fibre, while the other did not contain any PP fibre. Heating temperatures were set to room temperature (RT), 120, 200, 300 and 400 °C, considering the temperatures for the melting and burnout of the PP fibre. After heating and cooling, compression tests were carried out on the concrete specimens to measure the modulus of elasticity and Poisson's ratio. Pore size distribution was measured using the fragments created by the compression tests. Air permeability was estimated by measuring the pore size distribution. It was found that melting and burnout of the fibre did not affect the compressive strength and modulus of elasticity but the Poisson's ratio of the specimens containing fibres increased at 400 °C. The effect of melting and burnout of fibre on pore volume and air permeability is quite small. If it is assumed that micro-cracks affected the air permeability, it is expected that high strength concrete with a large fibre content should create many micro-cracks at high temperature, leading to an increase of air permeability.     

Influence of test specimen on experimental characterization of timber–concrete composite joints

Load-carrying capacity of timber–concrete composite joints is usually evaluated using shear tests, which still lack specific standards. Regulations EN 26891 [1] and ASTM D 5652 [2] are usually used, both for timber joints, or EUROCODE 4 [3] for steel–concrete composite joints. Questions about test execution and arrangement of specimens are frequent and recurrent [4], [5], [6]. Steel–concrete composite structures already have a standard shear test for joints (push-out), described in Johnson and Anderson [7]. These authors also discussed the many differences in the results of shear tests because of differences in test methods before EUROCODE 4 [3] standardization.This paper presents some questions about the arrangement of test specimens for shear tests in timber–concrete joints. An experimental program was performed for this reason. The aim of the work was to compare shear test results using two different series of specimens most utilized in a review of the literature: the push-out type with concrete center and timber sides and the push-out type with timber center and concrete sides. 8.0, 10.0 and 12.5 mm diameter corrugated bars were used as connectors. Eucalyptus grandis Brazilian hardwood timber glulam was used. Two-component epoxy adhesive was used to glue the connectors into the timber. Average cylinder compressive strength of the concrete was 25 MPa (28 days old). Reinforcement was 6.0 mm diameter 500 MPa-yield-stress corrugated bars.The results showed that test specimen arrangement influenced the strength and deformation characteristics of timber–concrete composite joints. The specimen with the best shear strength was the concrete–wood–concrete type, similar to those used in steel–concrete composite structures. Since the arrangement of test specimen is an important factor in joint tests, it is recommended that further efforts be made towards standardization.     

Investigation of effects of aggregate size on the fracture behavior of high performance concrete by acoustic emission

As an important component of concrete, aggregate has a significantly influence the fracture behavior of high performance concrete. In this paper, the effects of aggregate size distribution on the fracture properties of high performance concretes with strength ranging from 50 MPa to 80 MPa were investigated under three-point-bending tests. Acoustic emission (AE) technique with three-dimensional orientation feature was applied to study the effect of maximum aggregate size (d_max) on fracture properties and the facture process zone (FPZ) at the crack tip. The results showed that the fracture energy of concrete increased with the increase of d_max. The larger the size of the aggregate, the more significant the deflection of propagating crack and the greater the FPZ can form. That AE hits had a very good relation indicating its potential to measure inner damage of concrete during cracking.     

Testing of full-scale concrete bridge deck slabs reinforced with fiber-reinforced polymer (FRP) bars

This paper presents an experimental study investigating the behavior of FRP-reinforced concrete bridge deck slabs under concentrated loads. A total of eight full-scale deck slabs measuring 3000-mm long by 2500-mm wide were constructed. The test parameters were: (i) slab thickness (200, 175 and 150 mm); (ii) concrete compressive strength (35–65 MPa); (iii) bottom transverse reinforcement ratio (1.2–0.35%); and (iv) type of reinforcement (GFRP, CFRP, and steel). The slabs were supported on two parallel steel girders and were tested up to failure under monotonic single concentrated load acting on the center of each slab over a contact area of 600 × 250 mm to simulate the footprint of sustained truck wheel load (87.5 kN CL-625 truck). All deck slabs failed in punching shear. The punching capacity of the tested deck slabs ranged from 1.74 to 3.52 times the factored load (P_f) specified by the Canadian Highway Bridge Design Code (CHBDC) CAN/CSA S6-06. Besides, the ACI 440.1R-06 punching strength equation greatly underestimated the capacity of the tested slabs with an average experimental-to-predicted punching capacity ratio (V_exp/V_pred) of 3.17.     

Impact resistance of steel fibrous concrete containing fibres of mixed aspect ratio

The paper presents results of an investigation conducted to study the impact resistance of steel fibre reinforced concrete containing fibres of mixed aspect ratio. An experimental investigation was planned in which 108 plain concrete and SFRC beam specimens of size 100 × 100 × 500 mm were tested under impact loading. The specimen incorporated three different volume fractions i.e. 1.0%, 1.5% and 2.0% of corrugated steel fibres. Each volume fraction incorporated mixed steel fibres of size 0.6 × 2.0 × 25 mm and 0.6 × 2.0 × 50 mm in different proportions. The drop weight type impact tests were conducted on the test specimens and the number of blows of the hammer required to induce first visible crack and ultimate failure of the specimen were recorded. The results are presented in terms of number of blows required as well as impact energy at first crack and ultimate failure. It has been observed that concrete containing 100% long fibres at 2.0% volume fraction gave the best performance under impact loading.     

Effect of Granulated Blast Furnace Slag and fly ash addition on the strength properties of lightweight mortars containing waste PET aggregates

In this work, the effect of Granulated Blast Furnace Slag (GBFS) and fly ash (FA) addition on the strength properties of lightweight mortars containing waste Poly-ethylene Terephthalate (PET) bottle aggregates was investigated. Investigation was carried out on three groups of mortar specimens. One made with only Normal Portland cement (NPC) as binder, second made with NPC and GBFS together and, third made with NPC and FA together. The industrial wastes mentioned above were used as the replacement of cement on mass basis at the replacement ratio of 50%. The size of shredded PET granules used as aggregate for the preparation of mortar mixtures were between 0 and 4 mm. The waste lightweight PET aggregate (WPLA)–binder ratio (WPLA/b) was 0.60; the water–binder (w/b) ratios were determined as 0.45 and 0.50. The dry unit weight, compressive and flexural–tensile strengths, carbonation depths and drying shrinkage values were measured and presented. The results have shown that modifying GBFS had positive effects on the compressive strength and drying shrinkage values (after 90 days) of the WPLA mortars. However, FA substitution decreased compressive and flexural–tensile strengths and increased carbonation depths. Nevertheless a visible reduction occurred on the drying shrinkage values of FA modifying specimens more than cement specimens and GBFS modified specimens. The test results indicated that, GBFS has a potential of using as the replacement of cement on the WPLA mortars by taking into consideration the characteristics. But using FA as a binder at the replacement ratio of 50% did not improve the overall strength properties. Although it was thought that, using FA as binder at the replacement ratio of 50% for the aim of production WPLA concrete which has a specific strength, would provide advantages of economical and ecological aspects.     

Mechanical properties of natural fibers reinforced sustainable masonry

This research evaluates the physical and mechanical properties of Portland cement masonry blocks reinforced with lechuguilla natural fibers, that were lightened with 2-l bottles of polyethylene terephthalate.A concrete mix was designed for a target compressive strength of 16 MPa at 28 days, and slump of 70 mm. Masonry concrete blocks with dimensions of 730 × 340 × 130 mm were produced for two different fiber lengths (25 and 50 mm) and with fiber contents of 0.25%, 0.50%, 0.75% and 1.0%.Based on the obtained results, it was found that as the aspect ratio decreases the compressive strength increases and that the use of natural fiber (Vf = 0.5–0.75%) improves masonry post-cracking features, showing a ductile behavior and generating a uniform cracking pattern in the longitudinal sides of the blocks.     

Residual compressive behaviour of pre-heated high-performance concrete with blast–furnace–slag

An experimental program was designed and carried out to study the residual compressive behaviour of high-performance concrete (HPC) with blast–furnace–slag (BFS) at elevated temperatures ranging from 20 to 800 °C. The residual cube compressive strength is examined and the relationship between the residual compressive strength and temperature is investigated based on the heated cube specimens (100×100×100 mm³) tested on a universal test machine. In addition, on the basis of the heated prism specimens (100×100×300 mm³) tested on an electro-hydromantic rigidity servo test machine, the complete stress–strain curves are obtained, and the effects of temperatures on the residual prism compressive strength, the strain, and the elastic modulus etc are analysed. An approximate formula for the stress–strain relationship of HPC–BFS after exposure to temperatures is proposed.     

Estimating geometrical and mechanical REV based on synthetic rock mass models at Brunswick Mine

This paper uses a case study from Brunswick Mine in Canada to determine a representative elementary volume (REV) of a jointed rock mass in the vicinity of important underground infrastructure. The equivalent geometrical and mechanical property REV sizes were determined based on fracture systems modeling and numerical experiments on a synthetic rock mass. Structural data collected in massive sulphides were used to generate a large fracture system model (FSM), 40 m×40 m×40 m. This FSM was validated and subsequently sampled to procure 40 cubic specimens with a height to width ratio of 2 based on sample width from 0.05 to 10 m. The specimens were introduced into a 3D particle flow code (PFC3D) model to create synthetic rock mass (SRM) samples. The geometrical REV of the rock mass was determined based on the number of fractures in each sampled volume (P₃₀) and the volumetric fracture intensity (P₃₂) of the samples. The mechanical REV was estimated based on the uniaxial compressive strength (UCS) and elastic modulus (E) of the synthetic rock mass samples.The REV size of the rock mass was determined based on a series of statistical tests. The T-test was used to assess whether the means of the samples were statistically different from each other and the F-test to compare the calculated variance. Finally, the coefficient of variation, for the synthetic rock mass geometrical and mechanical properties, was plotted against sample size. For this particular site the estimated geometrical REV size of the rock mass was 3.5 m×3.5 m×7 m, while the mechanical property REV size was 7 m×7 m×14 m. Consequently, for engineering purposes the largest volume (7 m×7 m×14 m) can be considered as the REV size for this rock mass.     

Study of surface subsidence above an underground opening using a trap door apparatus

Physical model simulations have been performed to determine the effects of underground opening configurations on surface subsidence under super-critical conditions. This paper indicates the importance of the main factors that control the extent of subsidence produced on the surface and determines the effects of geometry of underground openings on the angle of draw, the maximum subsidence and the volume of the subsidence trough. A trap door apparatus with the test area of 95 × 95 cm² has been fabricated to perform the scaled-down simulations of surface subsidence. Gravel is used to represent the overburden in order to exhibit a cohesionless frictional behavior. In plan view the excavation dimensions are sufficient to induce maximum possible subsidence. The findings can be used to evaluate the subsidence profile for tunnels and caverns in soft ground. The results show that the angle of draw and the maximum subsidence are controlled by the width (W), length (L), height (H) and depth (Z) of the underground openings. The angle of draw and maximum subsidence increase with increasing L/W ratio and tends to approach a limit when L/W equals 3. For the same L/W ratio and H/W ratio, increasing the Z/W ratio reduces the angle of draw and maximum subsidence. The volume of the subsidence trough increases with increasing H/W ratio and L/W ratio. The width of the subsidence trough can be represented by sets of empirical relations. The relation between opening depth and subsidence trough developed by Rankin (for cohesionless soils) is in good agreement with most physical model results for deep openings (Z/W = 2–4), while for Z/W = 1, the predicted trough width is less than the physical model simulation. The volume of the subsidence trough is largest for Z/W = 2.5 and for H/W = 0.6, and is about 60% of volume of the underlying opening.